Vehicles include a power source, such as an internal combustion engine, that drives wheels through a transmission. The transmission is actuable to select one of a variety of gear ratios through which the wheels are driven. In the case of a manual transmission, the transmission is manually manipulated by the vehicle operator to achieve the desired gear ratio. In the case of an automatic transmission, the transmission is electronically controlled by a controller to achieve the desired gear ratios.
Implementation of either a manual transmission or an automatic transmission involves the use of friction devices. These friction devices include single-plate clutches, multi-plate clutches, cone clutches and band brakes. The friction devices enable selective interconnection of two rotating members. In general, friction devices include separate friction surfaces that selectively engage and disengage to form a link between the two rotating members.
In the case of a manual transmission, a manually operated friction device (i.e., clutch) is positioned between the power source and the transmission. The friction device enables smooth and progressive engagement between the power source and transmission for taking up initial drive and rapid disengagement for gear selection. In the case of an automatic transmission, a series of friction devices (e.g. single-plate clutches, multi-plate clutches, cone clutches and band brakes) are incorporated to enable gear selection. Other friction devices may be included such as an output clutch.
Heat is generated within the friction devices as a result of the progressive engagement of the friction surfaces. High temperatures and excessive heat generation adversely effect the friction device components. Traditional heat dissipation or cooling schemes are limited in effect. For example, one scheme limits friction device cooling to a predetermined time after engagement. That is to say, cooling fluid flows through the friction device for a predetermined time after actuation. Such cooling control is inadequate as it is independent of the actual heat generated within the friction device or the temperatures experienced by the friction device components.